To understand the gene expression of CAP18 (18-kDa cationic antibacterial protein), a member of cathelicidins, we evaluated mRNA and protein expression of CAP18 using human bone marrow cells and mature neutrophils. Northern blot analysis revealed that CAP18 mRNA was expressed more abundantly in bone marrow cells than mature neutrophils, whereas Western blot analysis indicated that CAP18 protein was more abundant in mature neutrophils than bone marrow cells. Consistent with this, in situ hybridization using bone marrow cells demonstrated that the expression of CAP18 mRNA was neutrophil lineage-specific and was observed primarily in myelocytes (>95%) with limited expression in more immature cells (promyelocytes) and mature cells (metamyelocytes, band cells, and segmented neutrophils). Furthermore, immunohistochemical study indicated that, coincident with the increase of CAP18 mRNA levels, CAP18-positive cells increased markedly at myelocyte stage, and the increased levels remained almost constant (>95%) in metamyelocytes, band cells, and segmented neutrophils, although the mRNA levels were remarkably reduced in these cells. Together these observations indicate that CAP18 gene transcription likely occurs lineage- and stage-specifically at the myelocyte stage of neutrophil maturation in the bone marrow and results in the synthesis and cytoplasmic accumulation of CAP18, which is present in the subsequent stages of neutrophil maturation.